The 57-year-old singer died in April from a self-administered Fentanyl overdose and the FBI, the DEA and the Carver County District Attorney's Office are trying to discover how he acquired the opioid, which has a high risk for addiction and dependence.

TMZ reports that authorities are ''scrutinising Prince's primary physician Dr. Michael Schulenberg and renowned addiction specialist Dr. Howard Kornfeld, to determine if either of them crossed the legal line by writing prescriptions to Prince''.

Prince, whose full name was Prince Rogers Nelson, was found unresponsive in an elevator at his Paisley Park home in Chanhassen, Minnesota on April 21 from a ''self-administered Fentanyl overdose''.

It was previously claimed that Prince had been addicted to the painkiller Percocet for years because of problems with his hips.

Prince - who had used a cane to help him get around for many years - had long needed a double hip replacement but reportedly turned it down on numerous occasions as he was worried it would require a blood transfusion, which would have conflicted with his beliefs as a Jehovah's Witness.

Dr. Schulenberg was at Prince's home when his body was found, and says it was in order to give him some test results.

Dr Kornfeld, a national authority on opioid addiction treatment, had been approached by Prince's team about meeting with the singer about his addiction to painkillers but could not leave California immediately and sent his son Andrew ahead of him.

However, when Andrew arrived at Prince's Paisley Park estate on April 21, Prince was nowhere to be found and he and two of the star's employees eventually found him unresponsive in an elevator.

According to Kornfeld's lawyer, William Mauzy, Andrew said the employees screamed when they found Prince and ''were in too much shock'' to call 911 so he phoned for an ambulance.